1. Field of the Invention
The invention relates to the field of tank systems for etching semiconductor parts and more particularly to such systems having provision for circulating the treatment solutions and for adjusting the temperature of the treatment solutions.
2. Prior Art
The semiconductor industry uses etching solutions for cleaning wafers, removing unwanted layers from wafers, stress relief of wafers and stock removal from wafers. Stock removal may be uniform or may be non-uniform in a pattern determined by previous processing. These etching solutions are often referred to by reference to their intended purpose. These solutions react exothermically with a material to be removed from the wafers. The rate of material removal in most etching processes is strongly dependent on solution composition, concentration and temperature. Thus, composition nonuniformity and temperature variations make wafer-to-wafer and lot-to-lot process control extremely difficult. This is especially true where only part of the thickness of a layer is to be removed.
In many etching processes the etching solution must be circulated within the tank in order to assure compositional uniformity and temperature stability by enabling a solution cooling system to remove the heat produced by the exothermic reaction. Mere placement of cooling coils within a prior art unobstructed or non-compartmentalized treatment tank does not produce the required degree of temperature stability and composition uniformity.
Mechanical stirring of the solution is complicated because the corrosiveness of the etching solutions severely limits materials which can be used in such equipment. In addition, mechanical stirring can produce non-uniform circulation and often leaves localized eddies and backwaters where little circulation or mixing occurs. Prior art circulation systems intended to induce uniform solution temperature and composition have used pumps, propellers, air bubblers and such for stirring the solution and inducing it to flow past cooling coils. In a further attempt to obtain uniform, repeatable processing results some prior art systems also revolve either the wafer rack containing the wafers or the wafers within the wafer rack.
U.S. Pat. No. 3,964,957 to Walsh discloses a treatment solution circulation system which uses an air bubbler as a pump. The tank for this system has a complicated, convoluted structure in order to prevent the air bubbles from reaching the wafers and producing bubble tracks thereon.
Bubble tracks are areas of a wafer which are treated to a different degree (usually a lesser degree) than the rest of the wafer as a result of bubbles thereon during at least part of the treatment period. Such bubbles change the degree of treatment by preventing solution contact with the surface of the wafer or by preventing that portion of the solution in contact with the wafer surface from having its active ingredients replenished as they are consumed by chemical reactions. These bubbles sometimes remain stationary on the wafer, but can also migrate up the vertical surface of a vertically oriented wafer. Migrating bubbles change the degree of treatment along their entire path, thus the term bubble tracks.
Prior art systems present extreme maintenance problems which can cause downtime. They also consume energy and involve substantial capital, running and maintenance costs. Downtime for equipment repairs is usually very costly because of interrupted product flow and the semiconductor industry is extremely cost conscious. Consequently, any reduction in maintenance downtime, energy consumption and capital, running and maintenance costs is extremely desirable.
The semiconductor industry has been trying to simultaneously increase process repeatability and reliability while reducing complexity and cost. Each of these prior art systems partially sacrifices one or more of these objectives in an attempt to meet the others.